Epoxy and polyurethane polymer systems are popularly used in a variety of applications like protective coatings, industrial coatings, composites, and adhesives. See, e.g., Edwards et al., J. Coating. Tech. Res. 2005, 2(7), 517-527; Harkal et al., J. Coating. Tech. Res. 2010, 7(5), 531-546. The presence of intermolecular bonds within the polyurethane matrix results in high mechanical strength, toughness, and abrasion and chemical resistance. See, e.g., Harkal et al., J. Coating. Tech. Res. 2010, 7(5), 531-546; Chattopadhyay et al., Prog. Polym. Sci. 2007, 32(3), 352-418. Epoxies exhibit a versatile and convenient crosslinking chemistry, wherein the oxirane rings undergo ring opening reaction with amines. See, e.g., Edwards et al., Prog. Org. Coat. 2006, 57(2), 128-139; Royappa et al., J. Appl. Polym. Sci. 2004, 91(2), 1344-1351. The rate of the curing reaction depends on the type of amine crosslinker and the relative stoichiometric ratio of amine to epoxy. The resultant coatings show excellent corrosion resistance and adhesion to substrates. Glycidyl carbamate (GC) functional resins combine the properties of polyurethanes with the convenience of epoxy-amine chemistry.
GC resins can be synthesized by the reaction of isocyanates with glycidol to form carbamate linkages (—CO—NH—). See, e.g., Edwards et al., J. Coating. Tech. Res. 2005, 2(7), 517-527; Edwards et al., Prog. Org. Coat. 2006, 57(2), 128-139; Chattopadhyay et al., Prog. Org. Coat. 2008, 63(4), 405-415; Chattopadhyay et al., Prog. Org. Coat. 2009, 64(2), 128-137; Harkal et al., J. Coating. Tech. Res. 2010, 7(5), 531-546; Harkal et al., Progress in Organic Coatings 2012, 73(1), 19-25; Chattopadhyay et al., Prog. Org. Coat. 2009, 66(1), 73-85; Harkal et al., J. Coating. Tech. Res. 2011, 8(6), 735-747. The resultant resin combines the strength and abrasion resistance of polyurethanes with convenient epoxy-amine crosslinking chemistry. A typical formulation with GC resin consists of the carbamate resin and an amine crosslinker. When cured, the resultant thermoset possesses excellent mechanical strength and chemical resistance, and can be easily altered depending on the application by changing the type and amount of amine crosslinker in the GC formulations. In the past, Webster et al. have studied viscosity modification of GC resins (see Harkal et al., J. Coating. Tech. Res. 2010, 7(5), 531-546), sol-gel modified systems (see Chattopadhyay et al., Prog. Org. Coat. 2008, 63(4), 405-415; Chattopadhyay et al., Prog. Org. Coat. 2009, 64(2), 128-137), and water dispersible resins (see Harkal et al., J. Coating. Tech. Res. 2011, 8(6), 735-747) containing polyethylene glycol. Although isocyanate is used to synthesize GC resins, absence of free isocyanates in the final coating formulation is expected to greatly reduce hazards associated with spraying unreacted isocyanates.
This invention shifts the focus onto synthesizing and formulating coatings for marine fouling-release (FR) applications. Biofouling is the undesirable attachment and colonization of aquatic organisms, like microalgae and barnacles, on submerged surfaces. See Callow et al., Nat. Commun. 2011, 2, 244; Yebra et al., Prog. Org. Coat. 2004, 50(2), 75-104. The process of biofouling can be commonly explained as formation of an organic conditioning layer, followed by accumulation of microorganisms, like bacteria and algal species, and finally attachment and growth of macrofoulants, like barnacles and mussels. See Yebra et al., Prog. Org. Coat. 2004, 50(2), 75-104; Magin et al., Mater. Today 2010, 13(4), 36-44. Common disadvantages of biofouling include increase in drag and, therefore, increase in fuel consumption, reduction in the speed of the vessel, and migration of aquatic species to non-native environments. See Callow et al., Nat. Commun. 2011, 2, 244; Schultz, Biofouling 2007, 23(5), 331-341; Sommer et al., Biofouling 2010, 26(8), 961-972. Furthermore, the economic impact of biofouling also cannot be ignored; combating biofouling can cost as high as a billion dollars annually. See Sommer et al., Biofouling 2010, 26(8), 961-972.
Two main technologies have been introduced to combat biofouling. Traditional antifouling (AF) coatings contained tin, copper, or organic biocides, which would leach out over time and completely prevent the attachment of organisms. See Sommer et al., Biofouling 2010, 26(8), 961-972. Although highly effective, the potentially toxic nature of the leachates has led to the replacement of AF coatings with “safer” FR coatings. See Callow et al., Nat. Commun. 2011, 2, 244; Sommer et al., Biofouling 2010, 26(8), 961-972. FR coatings allow attachment of organisms, but the weak bond can be easily broken by hydrodynamic forces. See Sommer et al., J. Coating. Tech. Res. 2011, 8(6), 661-670; Bodkhe et al., J. Coating. Tech. Res. 2012, 9(3), 235-249. Commercially available FR coatings are typically based on low modulus (soft) silicone elastomers, which lack the required mechanical strength and adhesion to the substrate, making them less viable in long-term applications. See Sommer et al., J. Coating. Tech. Res. 2011, 8(6), 661-670; Bodkhe et al., J. Coating. Tech. Res. 2012, 9(3), 235-249.
To overcome the short comings of the commercial FR coatings, Webster et al. developed a self-stratified siloxane-polyurethane (SiPU) coating system. See, e.g., Sommer et al., J. Coating. Tech. Res. 2011, 8(6), 661-670; Bodkhe et al., J. Coating. Tech. Res. 2012, 9(3), 235-249; Ekin et al., J. Comb. Chem. 2007, 9(1), 178-188; Ekin et al., J. Coating. Tech. Res. 2007, 4(4), 435-451; Majumdar et al., Thermoset Siloxane-Urethane Fouling Release Coatings. ACS Publications: 2007. A typical coating formulation with SiPU system comprises an isocyanate, a polyol, and a difunctional high MW siloxane (APT-PDMS). Upon curing, the siloxane component stratifies to form the outer low surface energy layer, while the PU matrix provides mechanical strength and improved adhesion to the substrate. But recent concerns regarding the presence of unreacted isocyanate groups in 2K coating formulations has necessitated further research to find “safer” alternatives to the PU matrix. GC resin technology shows potential to reduce the hazards of isocyanates in 2K coating formulations. Therefore, GC systems are potential substitutes to make self-stratified FR coatings.